An Etymological Dictionary of Astronomy and Astrophysics

In → geometric optics, a condition for eliminating → spherical aberration and → coma in an → optical system. It is expressed by the relationship: sin u’/U’ = sin u/U, where u and U are the angles, relative to the → optical axis,
of any two rays as they leave the object, and u’ and U’ are the angles of the same rays where they reach the image plane. A system which satisfies the sine condition is called → aplanatic.

See also: Named after Ernst Karl Abbe (1840-1905), a German physicist; → sine; → condition.